The present invention relates to a semiconductor device, more particularly, to a thin film transistor, for an electro-optical device and a method of manufacturing the same. Even more particularly, the present invention relates to a semiconductor circuit having a plurality of thin film transistors (TFT) for an electro-optical device. A TFT according to the present invention is formed on either an insulating substrate made of glass or the like or a semiconductor substrate made of single crystal silicon or the like. The present invention especially relates to a semiconductor circuit having a matrix circuit operating at a low speed and peripheral circuits operating at a high speed for driving the same such as a monolithic active matrix circuit (used for a liquid crystal display device or the like).
Recently, techniques for forming transistors utilizing a semiconductor thin film (thin film transistors (TFT)) on a substrate made of glass or quartz have researched. Especially, techniques utilizing amorphous silicon as a semiconductor thin film has been put in practical use and are applied to active matrix type liquid crystal display devices or the like.
However, a TFT utilizing amorphous silicon has a problem in that its has low characteristics. For example, the characteristics of a TFT utilizing an amorphous silicon film is too low to provide high display performance of an active matrix type liquid crystal display device.
A technique for forming a TFT using a crystalline silicon film obtained by crystallizing an amorphous silicon film is known wherein an amorphous silicon film is transformed into a crystalline silicon film by performing a heating process or laser irradiation after forming an amorphous silicon film. In general, a crystalline silicon film has a polycrystalline structure or a microcrystalline structure. A TFT formed by a crystalline silicon film provides characteristics much higher than those of a TFT formed by an amorphous silicon film. In mobility which is one of parameters for evaluating the characteristics of TFTs, a TFT utilizing an amorphous silicon film has mobility of 1-2 cm.sup.2 /Vs or less while the mobility of a TFT utilizing a crystalline silicon film can be about 100 cm.sup.2 /Vs or more.
However, a crystalline silicon film has many problems originating from grain boundaries because it has a polycrystalline structure. For example, a severe limitation is put on the withstand voltage of a TFT by the presence of carriers which move through grain boundaries. Further, the characteristics of a TFT can be changed or deteriorated, when it is operated at a high speed. In addition, the presence of carriers moving through grain boundaries increases leakage current from a TFT when it is turned off.
In order to form an active matrix type liquid crystal display device with a high level of integration, it is desirable to form not only the pixel region but also the peripheral circuits on a single glass substrate. In this case, since several hundred thousands of pixel transistors provided in the form of a matrix are driven, the TFTs provided in the peripheral circuits must be capable of handling a high current.
In order to provide a TFT capable of handling a high current, a structure having a large channel width must be employed. However, a TFT utilizing a polycrystalline silicon film or microcrystalline silicon film is not practical even with a wide channel width because of the problem with the withstand voltage. Further, it is impractical from other points of views such as significant fluctuations in thresholds.
Also, researches are made on insulated gate type semiconductor devices having an active layer (also referred to as active region) in the form of a thin film on an insulating substrate. Especially, researches are active on insulated gate type transistors in the form of a thin film, i.e., the so-called thin film transistor (TFTs). They are formed on a transparent insulating substrate and are used for pixel control and for driving circuits in display devices such as liquid crystal display devices having a matrix structure.
Semiconductor thin films forming TFTs include amorphous silicon semiconductors and crystalline silicon obtained by crystallizing amorphous silicon semiconductors by heating and laser irradiation. TFTs utilizing such amorphous silicon thin film and crystalline silicon thin films are called amorphous silicon TFTs and crystalline silicon TFTs, respectively. Since the electrical field mobility of an amorphous semiconductor is generally low, such a semiconductor can not be used for TFTs which must be operated at a high speed. In order to manufacture circuits having higher performance, research and development are active on crystalline silicon TFTs.
Since the electrical field mobility of a crystalline semiconductor is higher than that of an amorphous semiconductor, a crystalline semiconductor can be operated at a higher speed. A CMOS circuit can be formed from crystalline silicon because not only an NMOS TFT but also a PMOS TFT can be obtained from crystalline silicon. In the field of active matrix type liquid crystal display device, a device having the so-called monolithic structure is known in which not only the active matrix circuit portion but also peripheral circuits (drivers and etc.) are formed using CMOS crystalline TFTs.
FIG. 10 shows a monolithic type active matrix circuit used in a liquid crystal display device. A column decoder 1 and a line decoder 2 are formed on a substrate 7 as a peripheral driver circuit. A multiplicity of pixel circuits 4 including transistors and capacitors are formed in a matrix region 5 wherein a multiplicity of pixels are provided in the form of a matrix, and the matrix region and the peripheral circuit are connected to each other by wirings 5 and 6. The TFTs used in the peripheral circuit must be able to operate at a high speed and the TFTs used in the pixel circuit must have a low leakage current. The device having such characteristics contradictory to each other must be formed on the same substrate simultaneously.
However, TFTs manufactured by the same process have similar characteristics. Although crystalline silicon can be obtained by crystallization using a laser (laser annealing), if crystalline silicon obtained by laser crystallization is used for both of the TFTs for the matrix region and the peripheral driving circuit region, the TFTs will have similar characteristics. As a result, it will be difficult to achieve both of the low leakage current characteristic required for the pixel circuit and the high mobility characteristic required for the peripheral driving circuits.